4 Thomson, Notes on Arsenic. 



metallic ring, and the larger mirrors are more evenly 

 deposited. 



Influence of nitrous compounds on the estimation of 

 minute quantities of arsenic. 



I observed in making tests by the electrolytic as 

 compared with the Marsh-Berzelius method that in some 

 cases considerably larger mirrors were obtained by the 

 latter than by the former, and on investigation it turned 

 out that this difference was due to the fact that all the 

 nitrous compounds used for the destruction of the organic 

 matter had not been removed. To be certain of the 

 results by the electrolytic process it becomes necessary 

 that the sulphuric acid should be evaporated to its fuming 

 point, diluted with twice its volume of water and again 

 evaporated to the fuming point, and this should be again 

 repeated to ensure the elimination of all of the nitrous 

 compounds, so that the proper size and depth of arsenic 

 mirrors may be obtained by the electrolytic process in the 

 time allowed for the test. With the Marsh-Berzelius 

 method the presence of some nitrous compounds does 

 not so materially affect the result. 



With a view of studying the influence of nitric acid 

 in the electrolytic apparatus, a dilute solution equivalent 

 to half a c.c. of the ordinary strong acid of commerce 

 (0'48 grm. HNO3) was added separately to an electrolytic 

 apparatus — the first with a lead, the second with 

 a zinc, and the third with a graphite kathode. This 

 acid was gradually converted into ammonia in each 

 apparatus, the amount of which was then determined. 

 A current of 3 amperes was employed, and each apparatus 

 was worked in series. After 45 minutes the ammonia 

 in the solutions in the various kathode chambers was 

 determined, and the following results obtained : — 



